A large coil provides an increased search speed and great depth. For the Fisher F2-F4 users, there exist not only original coils: the alternative manufacturers also produce ones for these machines. See the photo: Mars Goliath 15” coil for Fisher F2-F4.
This is the coil from Mars MD (alternative manufacturer). It’s also produced for other metal detectors. Type: DD. Shape: round. Size: 15 inches (38cm).